1. Field of the Invention
This invention relates to methods of making improved electronic systems and circuit boards, and more specifically to methods of making improved electronic systems and circuit boards using heat-resistant composite materials. Various novel heat-resistant electronic systems, circuit boards, non-segregating solid reinforcing elements, and other products based on these methods are also disclosed.
2. Description of Related Art
Electronic systems with modern electronic circuits components or elements are used in almost every industry including manufacturing, servicing, banking, business, financial, medical, end weaponry, as well as in high-speed processors, cellular phones, satellite communication systems, deep-well equipment, jet engines, gas turbines, lap-top personal computers, nuclear reactors, and automobiles or other transportation vehicles. Users of these electronic systems continuously require larger, more powerful, and faster speeds, requiring such systems to posses and better and more processors, transistors, voltage regulators, memory, and other components.
Generally, electronic circuit components or elements are mounted e.g., by melting and solidifying a solder metal on plastic or ceramic circuits boards. Metallic lead wires or lines are provided on the circuit components for use in connecting these circuit components onto the circuit boards. These connecting lines must be as few in number and as short as possible to reduce the electrical resistances, which slow down the speed of the electronic systems. These metallic lines must also be rigid, strong, fatigue-resistant, creep-resistant, and thermally conductive to help dissipate heat. Excessive heat generation from, e.g., high electrical resistances, increases the system temperature, reduces the life of transistors, and lowers the mechanical strength and creep resistance of metallic lead wires, thereby causing run-away deterioration of electrical and thermal resistance, temperatures, transistor life, and lead wire mechanical strengths. The degraded electronic systems directly degrades the performance of any equipment containing such electronic systems.
In many electronic systems, thermal design already is the limiting factor. For example, to handle the heat of a high-power (15.4 W) TO-220 voltage regulator operating with a 233-MHz Pentium chip presents a formidable problem that requires a proper thermal solution without scraping the existing mother-circuit-board architecture. Pentium chips with even higher speeds are already in mass production.
An important consideration in the mechanical, thermal, and electrical design of a circuit board and an electronic system is the fact that many materials are used for the electronic circuit heat-resistant components, the plastic or ceramic circuit board, and the electronic system. In general, the electronic system has a metallic or plastic frame onto which the circuit board substrate is fixedly attached at a specific location thereon. The circuit board is used to electrically and physically connect a number of circuit components together. The circuit board substrate has a large number of through holes. Each electronic circuit component has a number of metallic lead wires embedded into and electrically separated by an encapsulant. All the metallic lead wires on each circuit component extend, and point in a common direction away, i.e., vertically downward as shown in FIG. 7, from the circuit component so that all the extending lead wires can be easily inserted simultaneously into selected through holes at given positions on the circuit board substrate. The inside surfaces of the through holes are coated with specific metal layers to facilitate the wetting and bonding of the inserted metallic lead wires.
Each of the many different materials on the circuit components, the circuit board substrate, and the system frame has a specific set of mechanical, electrical, and thermal properties, and a unique thermal expansion coefficient. At the contact area between any two different materials, there is an actual or a residual thermal expansion mismatch that generates thermal mismatch stresses. Specifically, when the electronic system changes in temperature, thermal mismatch stresses are generated between:
1) the metallic lead wires and their encapsulating plastic;
2) the metallic lead wires and the bonded metal layers on the through holes of the circuit board substrate; and
3) the circuit board substrate and the mounting frame of the electronic system.
The thermal mismatch stresses usually are highly localized and can be so very severe as to cause localized metal creep and fractures, or changes in electrical resistances and thermal conductances. Changes in these resistances and conductances are equally, if not more, damaging than other changes to the reliability and life of the circuit components, the circuit board, and the entire electronic system.
Hence, the circuit boards and the electronic systems must have radically improved lead wire connections, which may be fabricated by soldering, brazing, or welding methods. All these methods use molten metal alloys. Soldering metal alloys with melt temperatures below about 250xc2x0 C. are employed so that low-cost plastic circuit boards may be used. Brazing and welding metal alloys require melt temperatures respectively below and above about 800xc2x0 C. Such temperatures require ceramic circuit boards. Most such connections now are soldered joints that have:
1) high electrical resistance leading to wasteful heat generation, rise in temperatures, and reduced circuit component speed and life;
2) low thermal conductivity magnifying the problems in 1); and
3) inadequate mechanical strength of the lead wire connections particularly as to creep, fatigue, or shear, making all the bonded lead wires, the circuit board, or even the entire electronic system non heat-resistant, short-lived, and unreliable.
In this invention, the above-mentioned problems of prior-art composites are minimized by a unique, heat-resistant ceramic-reinforced composite material to be shown below.
For purposes of the present invention, a composite is any material that results when two or more materials, each having its own, usually different characteristics, are combined, giving useful properties for specific applications.
Further, when used in the present specification, a matrix is a material in which something is enclosed or embedded.
For purposes of the present invention uniform distribution of solid reinforcing elements in a composite matrix means that the concentration of the solid reinforcing elements in each unit of volume, e.g., cubic millimeter, of the solidified composite matrix is constant or substantially constant throughout the entire composite. In addition, a composite has a matrix component, the matrix component is generally characterized by the composite component that is in the majority. For example, a composite made from 20% by weight solid reinforcing elements and 80% by weight In is characterized as an In matrix composite.
Composites are important structural materials. Oftentimes composites are reinforced by suspending or embedding solid strengthening or reinforcing elements, such as, reinforcing powders, rods, sheets, weaves, or combinations thereof within the composite matrix. Generally, the solid reinforcing elements are rigid and temperature resistant and are thus used to make the entire composite matrix more rigid and temperature resistant. Many other benefits are achieved by reinforcing composites. For example, reinforced composites can be prepared which resist creep, fatigue, and tensile or shear fractures at temperatures which are close to the melting point of the composite matrix.
Reinforcing elements often segregate at corners, edges, and deep but narrow walls such as in a solder joint on a circuit board. Overcrowded reinforcing elements at certain segregated places, such as the bottom for heavier solid reinforcing elements or the top for lighter solid reinforcing elements, causes weakness in the composite matrix.
Reinforced composites are formed by adding solid strengthening or reinforcing elements to a liquid composite matrix followed by solidifying or freezing the mixture to provide a reinforced composite matrix which contains the solid strengthening or reinforcing elements embedded therein. Ideally, the solid reinforcing elements are uniformly distributed in the composite matrix to realize and optimize the desired performance of the reinforced composite matrix. However, it is extremely difficult, if not impossible, to achieve uniform distribution of reinforcing elements in a composite matrix.
The uniform distribution of the solid reinforcing elements in a liquid or solid composite matrix is a critical factor in achieving optimum composite performance. If the solid reinforcing elements are heavier than the composite matrix, they gravitationally segregate at the bottom of the liquid composite matrix during the solidification process. This segregation causes a non-uniform distribution of reinforcing elements in the composite matrix. This overcrowding also reduces the efficacy of these solid reinforcing elements and decreases the usefulness of the resulting reinforcing composite. Solid reinforcing elements float or segregate to the surface of the liquid matrix if they are lighter than the matrix.
Reinforcing elements segregation at corners, edges, and deep but narrow walls is also very common. Overcrowded reinforcing elements at certain segregated places, such as the bottom for heavier solid reinforcing elements or the top for lighter solid reinforcing elements, causes weakness in the composite matrix. In particular, if a composite matrix has too many solid reinforcing elements, it may be even weaker than a composite matrix without any reinforcement. This weakness results because the solid reinforcing elements are not sufficiently supported by, or connected to, the composite matrix which causes localized overstresses, and initiates voids and cracks in the matrix. Similarly, in areas of the composite matrix where solid reinforcing elements are underpopulated, the composite matrix is, of course, weak and not properly reinforced.
Proper reinforcement is also problematic in cases where a composite is narrow and deep, such as between two concentric cylinders, in narrow-clearance soldered joints on the circuit board. In this case the solder composite thickness between the inner and outer cylindrical walls may be less than 1 to 5 mils. Given these parameters, the gravitational segregation of solid reinforcing elements at localized spots may initiate premature composite failures.
Thus, an inferior composite can result because of the differing densities of the liquid composite matrix and the solid reinforcing elements. In particular, solid reinforcing elements sink when suspended in a lighter liquid composite matrix, and float when suspended in a heavier liquid composite matrix. In either case, the solid reinforcing elements segregate due to gravity, resulting in a non-uniform distribution of the solid reinforcing elements in the liquid composite matrix. Further, this non-uniform distribution pattern is carried over during the composite matrix solidification, e.g., freezing or resin polymerization of the composite matrix, resulting in undesirable segregation patterns of the solid reinforcing elements in the resultant solid composite matrix.
Different approaches, having varying degrees of success have attempted to overcome the deficiencies in the prior art reinforced composites. Specifically, a tedious and time-consuming process of hand packing reinforcing elements in a composite matrix has attempted to achieve the desired uniform distribution of the reinforcing elements. In particular, alternate sheets of composite matrix of a first thickness and solid reinforcing sheets or two-dimensional weaves of a second thickness may, for example, be hand-packed together, layer after layer, followed by liquid infiltration and freezing, pressing or thermal polymerization to form a resultant reinforced composite matrix. This process has several shortcomings, including non-uniform distribution of the reinforcing elements caused by shifting or settling of the packed material, irreproducibility of the packing results and excessive expense in forming the reinforced composite.
Another approach which has attempted to provide uniform distribution of reinforcing elements uses a process which suspends the solid reinforcing elements in a liquid or molten composite matrix. This suspension is then injected into and solidified in a mold causing the solid reinforcing element to be frozen in place. However, if the reinforcing elements are non-uniformly distributed in the liquid composite matrix prior to solidification or if the elements settle during the solidification, the final distribution of these elements in the solid composite is also non-uniform.
Thus, what is needed then are methods of making reinforced composites in which the solid reinforcing elements are uniformly, or substantially uniformly distributed in a composite matrix resulting in a composite matrix wherein the concentration of the solid reinforcing elements in each unit of volume, e.g., cubic millimeter, of the solidified composite matrix is constant or substantially constant throughout the entire composite.
In view of the prior art as a whole at the time of the present invention, it was not obvious to those of ordinary skill in the pertinent art how to provide for the heat-resistant, and fabricate reinforced composites for circuit board and electronic systems of the invention.
Accordingly, a method of making heat-resistant electronic systems, printed circuit boards, and soldered, brazed, or welded joints includes mixing a liquid metal matrix having a preselected liquid density with a plurality of solid reinforcing elements to provide a mixture. Each of the reinforcing elements has an average density substantially equal to the preselected liquid density thereby achieving a substantially uniform and stable distribution of the solid ceramic reinforcing elements in the liquid metal matrix. The liquid metal matrix is then frozen to preserve the uniform and stable distribution in the resultant solid composite and to make the electronic system, circuit board, or bonded joint heat-resistant. Products of the invention in various forms are also disclosed.
The invention accordingly comprises the features that will be exemplified in the description hereafter set forth, and the scope of the invention will be indicated in the claims.